In many applications it is not sufficient to be able to determine rotational angles over e.g. a half or full revolution. In order to carry out an unambiguous measurement of a plurality of revolutions, for example in an electricity meter or the spindle of a linear drive, it is also possible to use, in addition to electronic counting devices which are based on the signals of a simple angle measuring device, mechanical counters and transmissions. These have the advantages that they retain the counter reading even without a supply of energy and can even carry on counting without a supply of energy to the sensor system. This may be very important if in the event of a fault an inert machine continues to move while the sensor system is not functioning as a result of a power failure, or if the state of a machine is changed in the switched-off state.
In the text which follows, not only the state of a machine but in an equivalent fashion also the state of the device is mentioned, i.e. the state of the counter or of the transmission. Said state is described, for example, by means of the measurement of the individual rotational angles of all the wheels of a counter or gearwheels of a transmission.
Counters are also used in linear sensors, for example cable travel measuring devices, in order to expand their measuring range. As a rule, they do not permit any direct manual evaluation of the state because the compactness and complexity of the electronic detection of the counter reading in devices which can be read off manually is even greater than in the case of electronic evaluation, with the result that in this context there are either solutions which can be read electronically or solutions which can be read purely manually.
However, counters which can be read manually or mechanical counters are of interest in many fields because they operate with little maintenance and risk and independently of an additional power supply as well as permitting easy monitoring of the function or of the counter reading. Examples are, for instance, gas meters and water meters in which electrification would be conceivable most likely with battery operation or by means of energy harvesting, but associated with high costs and relatively high maintenance expenditure as well as, under certain circumstances, safety problems as a result of an increased risk of explosions or leakage. Even in the case of the abovementioned cable encoders in many applications there could be a measured value if the counting value could be read off not only purely electronically but also visually on the counter, even if the counter itself is without power.
The measurement of revolution angles in multi-digit counters and in transmissions of multi-turn rotary encoders currently entails high expenditure. As a rule, each counting wheel or each gearwheel of a device is equipped with an independent absolute-value angle sensor, with the result that e.g. a multiplicity of encoder disks, collimated light sources and precisely adjusted multi-track sensors is necessary, which additionally have to communicate with one another in order to eliminate ambiguities as a result of gear play etc. This constitutes considerable expenditure and makes such systems expensive.
The reason for this complicated type of design is the fact that conventional angle sensors can measure reliably only over short and constant distances. It is thus scarcely possible to read off the field of a magnetic code disk with a diameter of e.g. 1 cm from a distance of 1 cm with good accuracy because the magnetic field has dropped to very low values at this distance. If a plurality of magnet disks are to be arranged one behind the other, the signals would interfere with one another. Signals of classic optical encoders (transmissive or reflective with multi-track code disks) can also be read off from a relatively large distance only with a high adjustment complexity and precise optics because scattered light and measuring errors quickly predominate as a result of incorrect adjustment. However, this can be carried out to a limited degree. For example, EP143354 describes the superimposition of signals of two transmission stages. However, even more wide ranging multi-stage superimposition generates even more quickly increasing complexity of the design. In EP1457762 the distance between different code disks and the respective sensor is reduced by cylindrical elements with different radii to the distance which is customary for simple encoders. This requires a multiplicity of precisely fitting elements, requires them to be assembled to form complicated three-dimensional structures and does not permit any simple cascading to form higher step numbers with a kit system.